1. Field of the Invention
The present invention relates to a component mounting method and a component mounting apparatus for mounting electronic components and other components on a circuit substrate.
2. Description of Related Art
A conventional electronic component mounting apparatus shown in FIG. 10 includes an XY robot 1 adapted to position a head 3 having a nozzle 2. The XY robot 1 includes an X-axis robot 1a and Y-axis robots 1b-1 and 1b-2. An electronic component 5 is picked up from a component supply section 4 by the nozzle 2 and is moved with the head 3 to the mounting position on the circuit substrate 7 that is secured to an XY table 6. Then, the electronic component 5 is mounted onto the circuit substrate 7.
The operation of mounting the electronic component 5 on the circuit substrate 7 requires an enhanced degree of precision particularly when a number of components are to be arranged densely on the circuit substrate 7. For instance, if a 0.6 mm long and 0.3 mm wide micro-chip is displaced by more than ±25 μm from the right mounting position, stable bonding of an electrode of the chip and a corresponding land of the circuit substrate can not be achieved. However, the X-axis robot, the Y-axis robot, the center position of a camera CCD and various other parts of the machine frame become displaced due to temperature changes caused by environmental temperature changes, heat generated by a drive motor and other motors when the machine is made to start operating, and heat generated in sliding areas of the X-axis/Y-axis robots and so on. Consequently, the centers of components are displaced from the predetermined respective positions, thereby decrease the accuracy of mounting of the components.
This problem will be described in detail with reference to FIG. 11.
In FIG. 11, the X-axis robot 1a drives the head 3 along the X-axis by employing a motor 30, while the Y-axis robots 1b-1 and 1b-2 support the X-axis robot 1a at the respective ends thereof and drive the X-axis robot along the Y-axis (running in a direction perpendicular to the sheet showing FIG. 11). Normally, it is so arranged that one of the opposite ends of the X-axis robot 1a, e.g., the end where the Y-axis robot 1b-1 is located, is rigidly held, while the other end thereof, e.g., the end where the Y-axis robot 1b-2 is located, is able to slide in the X-direction.
The X-axis robot 1a receives heat generated by the motor 30, and frictional heat of the head 3 as well as that of the X-axis robot 1a itself as it slides on the Y-axis robots 1b-1 and 1b-2. The X-axis robot 1a expands as the temperature thereof rises. Particularly, the X-axis robot 1a expands in the X-direction at the side of the Y-axis robot 1b-2 by ΔX1. As a result of the expansion, the head 3 is displaced in the X-direction by the amount indicated by a broken line in FIG. 11. Accordingly, the nozzle 2 is displaced in the X-direction by ΔX2.
The Y-axis robots 1b-1 and 1b-2 and frames supporting the Y-axis robots also receive heat and expand in a similar way. Thus, the nozzle 2 is displaced by the combined effect of these expansions.
While the displacement of the nozzle 2 may be very small, it will not be negligible when it exceeds the required accuracy of positional arrangement.
Known electronic component mounting apparatus is normally provided with an automatic calibration feature for preventing degradation in the accuracy of positional arrangement due to temperature change. With this feature, whenever necessary, the electronic component mounting apparatus automatically carries out a calibrating operation as a function of the temperature change from a predetermined level or the time elapsed from the last calibration. The calibrating operation is carried out by attaching a calibration jig to the nozzle 2, the jig for determining the center of the nozzle 2 of the head 3, positioning the jig, picking up an image of the nozzle 2 with a camera, determining a displacement of the center of the nozzle 2 in terms of coordinates adopted for positioning, and updating the amount of offset (positional displacement due to temperature changes) relative to the mechanical original point of the apparatus.
However, the amount of offset relative to the mechanical original point of the apparatus as determined with the calibration jig is the sum of the amounts of offset of each of the units (including the head 3, the XY robots 1, and the camera) due to temperature changes. The amount of offset of each unit varies from a time of measurement to another so that errors arise. Consequently, the accuracy of positional arrangement of electronic component may be degraded even when the units are corrected in accordance with the amount of offset obtained by the calibrating operation.
Additionally, with the above-described scheme of automatic calibration, the nozzle, the camera and other components need to be subjected to a calibrating operation individually so that the productivity will deteriorate.
Still additionally, when the apparatus is made to start operating after a long pause spent for switching to a different type of devices, the amount of offset varies immediately because the temperature change is going on in the start-up phase. This means that the amount of offset obtained by the automatic calibration is reduced useless and the amount of the offset of each of the units needs to be determined anew.